Yeast organisms naturally transport a small number of certain homologous proteins to and sometimes through the plasma membrane as an essential contribution to cell surface growth and cell metabolism. As the cell buds as an incident of reproduction preparatory to formation of a daughter cell, additional proteins are required for formation of cell wall and plasma membrane as well as for metabolism. Some of these proteins are transported to the site of function, via a secretory pathway (1). Certain homologous proteins involved in the above processes are formed by translation in the rough endoplasmic reticulum (RER). During their biosynthesis, the signal peptide was thought to interact with a presumed particle called the signal recognition particle (SRP) which in turn recognizes a presumed RER membrane protein called SRP receptor or docking protein and subsequently the proteins are transferred across the RER membrane (2-4). Once proteins are segregated into the lumen of RER, they migrate to the Golgi apparatus and then to the vesicles. Fusion of the vesicles with the plasma membrane (by exocytosis) results in discharge of the vesicular contents (proteins) into the periplasmic space located between the plasma membrane and the cell wall. A small number of homologous proteins seems to be exported completely through the cell wall, such as .alpha.-factor and killer toxin (5,6).
Nonetheless, the entire set of secretion processes is not yet fully understood in yeast. The presumed SRP of yeast may not be identical to the counterpart in mammalian cells. In other words, some heterologous signals (other than yeast signals) may or may not bind the yeast SRP with the same fidelity as those homologous (yeast) signals in the expression of the heterologous genes. Likewise, it is still uncertain whether modifications of proteins in vivo (such as phosphorylation, glycosylation etc.) assist the translocation of secretory proteins along the secretion pathway.
It was contemplated that recombinant DNA technology could provide valuable assistance in answering the open questions about the secretory process in yeast organisms. Given its proven applicability in enabling such, and other, organisms to produce copious quantities of heterologous polypeptide products endogenously (see, e.g., 7 to 17), it was thought that such technology would be helpful in achieving appropriate manipulation of the yeast host so as to direct the secretion of heterologous protein in discrete, mature form. This has, in fact, been achieved and is the subject of copending application Ser. No. 438,236, supra. In that application is described the discovery that a heterologous protein, initially expressed as a preprotein with its native signal (human) or hybrid thereof, can be secreted by yeast as a mature protein.